The present invention generally relates to a molding die and, more particularly, is directed to a molding die for a disk substrate suitable for molding, for example., an optical disk, a magnetic disk, and the like.
Generally, an optical disk, a magnetic disk or the like, as seen in FIG. 1, made of synthetic resin is formed through injection molding with a die to which a thin metal plate called a stamper, carrying a signal concentrically or spirally recorded thereon, is fixed.
The eccentric accuracy of a disk, that is, the accuracy against the eccentricity of a disk, is determined on the basis of the synergism of three factors such as variations in accuracy in punching of a stamper center hole, backlash between the stamper and a stamper mounting holder, and backlash between the stamper holder and a body of the die. Accordingly, the eccentric accuracy is improved by reducing the variations and backlash of the three factors so that various disk standards are satisfied. For example, the eccentric accuracy allowable for the disk standards is, for example, .+-.70 .mu.m for a compact disk (CD), and .+-.50 .mu.m for a mini disk (MD).
As methods for manufacturing duplicate disks from plastic material, in addition to the aforesaid method using a stamper carrying a signal recorded thereon, there is known, for example, a method in which a signal is recorded directly on a die. This method, however, has a defect that it is difficult to cope with a change in the contents of software or exchange of software per se, and even if it is made possible to cope with such problems, the cost is increased correspondingly.
This is applied to the case where decomposition gas is generated from plastic material or the like or dirt/injury is generated in handling. Accordingly, the method of using a stamper is employed. In this case, however, there arises a problem that it is impossible to suppress the eccentric accuracy, for example, within 20 .mu.m in all cases.